Geon-Tae Hwang

Self‐powered flexible sensors represent indispensable components in tactile sensing and wearable electronic systems. In biological organisms, intracellular and extracellular ion transport underpin the precise perception, transmission, and processing of tactile stimuli. Inspired by these natural mechanisms, four types of self‐powered multifunctional sensors were developed based on the controlled motion of ions within cationic poly(diallyldimethylammonium chloride) and anionic sodium polystyrene sulfonate ionomers. The sensors exhibit a p–n junction configuration, where a depletion layer is established at the ionomer interface. Through the incorporation of 2D MXenes and 1D carbon nanotubes (CNTs), the electrical conductivity was optimized, yielding an open‐circuit voltage of approximately 75 mV and a short‐circuit current density of ∼67 µA cm ^{− 2} . The distinct rectification behavior (ratio ≈ 8.8) enables logic circuit functionality, while the tunable assembly of sensing units into arrays allows precise discrimination of compression, bending, and directional stress stimuli. Unlike conventional pressure‐sensing arrays, each unit in the present system displays unique sensing characteristics. This work offers a new paradigm for the rational design of high‐performance, self‐powered ionic sensors for next‐generation flexible and wearable electronics.

Lead-free bismuth sodium titanate (BNT)-based ceramics have attracted strong attention as environmentally benign dielectric materials for high-efficiency electrostatic energy-storage capacitors. A key challenge is that pristine BNT typically exhibits large hysteresis, high remnant polarization, and limited dielectric reliability, which restrict recoverable energy storage and efficiency under practical electric fields. Here, we present a focused mini-review of recent studies to clarify how composition design, phase boundary tuning, defect chemistry, and microstructural control collectively enable slim or pinched polarization-electric field (P-E) behavior and improved energy-storage functionality in BNT-related bulk ceramics. The reviewed outcomes consistently show that stabilizing relaxor states governed by polar nanoregions (PNRs), often via solid-solution engineering and secondary relaxor/antiferroelectric-like incorporation, suppresses irreversible switching and reduces hysteresis loss, while densification and grain-size control enhance electrical homogeneity and breakdown strength. In addition, defect-mediated tuning of oxygen vacancy-related complexes is highlighted as an independent lever to control relaxor ergodicity and polarization reversibility, providing a complementary route to slim-loop optimization. These insights are expected to guide integrated design strategies that couple phase/relaxor-state engineering with defect and microstructure optimization, accelerating the development of reliable, temperature-robust, lead-free dielectric capacitors based on BNT-related ceramics.

Magneto-mechano-electric (MME) energy harvesters have emerged as a promising solution for maintenance-free power generation in rapidly expanding Internet of Things (IoT) environments, where replacing or wiring batteries is impractical. MME devices convert weak alternating magnetic fields, ubiquitous around power infrastructures, into useful electrical energy through sequential magnetic, mechanical, and electrical transduction processes. This review summarizes recent advances across triboelectric-, piezoelectric-, and hybrid MME architectures. Triboelectric MME generators employing nano-engineered polymer surfaces, flash-induced surface modification, and nanoscale pattern replication demonstrate low-cost fabrication routes while achieving significantly enhanced voltage and current outputs. Piezoelectric MME systems based on Mn-doped PMN-PZT single crystals highlight strategies for improving mechanical quality factors and resonance-driven power generation. Further, hybrid MME designs that integrate piezoelectric and electromagnetic induction mechanisms enable high-power outputs exceeding tens of milliwatts, sufficient to operate multifunctional IoT platforms and charge practical energy-storage devices. Collectively, these studies illustrate a transition of MME harvesting technologies from laboratory concepts to application-ready self-powered systems. Future opportunities lie in broadband resonance design, modular harvester integration, advanced power management, and multi-source hybridization for robust long-term operation in real environments.

We report a flexible self‐charging capacitor system that integrates a triboelectric nanogenerator (TENG) with a flexible Pb(Zr _x Ti ₁ ‐ _x )O ₃ (PZT) ferroelectric ceramic capacitor by aerosol‐deposition (AD). The flexible PZT capacitor demonstrated high energy storage capabilities, achieving a recoverable energy density of 35.8 J cm ^{− 3} under an applied electric field of 2000 kV cm ⁻¹ , as determined from the measured polarization‐electric field (P‐E) hysteresis curve. To enhance triboelectric performance, a micro‐imprinting process is applied to the Al surface in the TENG, improving the contact surface area and output performance. Using biomechanical tapping as an input, the TENG produced a rectified open‐circuit voltage of 350 V, corresponding to an electric field of 1000 kV cm ⁻¹ across the PZT capacitor. The flexible self‐charging PZT capacitor exhibited a recoverable energy density of 5.9 J cm ⁻³ and a discharge power density of 16.0 MW cm ⁻³ . This improved power density is achieved through the combination of a simple micro‐patterning technique on the TENG and the annealing of the AD PZT film, which induced a slim ferroelectric P‐E hysteresis behavior. Our results suggest that this flexible self‐charging system holds significant promise for powering next‐generation flexible smart devices and high‐power electronics, offering a scalable and efficient approach to energy harvesting and storage.

The rapid growth of the Internet of Things infrastructure requires scalable and autonomous power solutions, as traditional battery and cable-based methods are impractical. Magneto-mechano-electric generators, which harvest ambient magnetic fields, present a promising alternative, though further research is needed to optimize their materials and designs for widespread deployment. In this study, we present a high-output magneto-mechano-electric generator utilizing Mn-doped single-crystalline Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) grown by the modified Bridgman method. The integration of Mn acceptor ions into the crystal structure significantly enhances the mechanical quality factor, which is crucial for efficient energy harvesting. Finite element analysis simulations were conducted to estimate the impact of acceptor doping on the performance of the magneto-mechano-electric generators. The Mn-doped magneto-mechano-electric generator achieved a root-mean-square power output of 106.2 mW under an induced alternating current magnetic field of 7 Oe, which is nearly 2.2 times higher than that of the undoped generator. The calculated root-mean-square power density of 35.8 mW/cm³ is approximately 8 times higher than that of the recently reported hybrid-type magneto-mechano-electric generator. The practical application of the Mn-doped harvester was demonstrated by powering multiple multifunctional Internet of Things sensors, including in a battery-free configuration. Real-time environmental monitoring data were successfully transmitted to smartphones, highlighting the potential of acceptor-doped piezoelectric PIN-PMN-PT crystals in developing high-performance, sustainable, and battery-free energy harvesting solutions, paving the way for their integration into a wide range of Internet of Things applications.

This comprehensive review of (K,Na)NbO ₃ -based perovskites explores design strategies, mechanisms, and applications in piezocatalysis and photo-piezocatalysis for sustainable environmental and energy solutions.